A Performance Evaluation ofthe Quality of Service

(Wireless LAN) StandardIEEE 802.11e

Muhammad Shahid Manzoor

September 30, 2008Master’s Thesis in Computing Science, 10 credits

Supervisor at CS-UmU: Thomas NilssonExaminer: Per Lindstrom

Umea UniversityDepartment of Computing Science

SE-901 87 UMEASWEDEN

Abstract

At present, the IEEE 802.11 standard is the most successful WLAN technology inthe world, due to its cheap cost and easy deployment. But it does not support QoS. TheQoS is referred as the quality of the data traffic over a network. Multimedia applicationsdemand consistent QoS support in terms of bandwidth and delay. Unfortunately, 802.11is incapable to fulfill these requirements due to deficiency in providing QoS support.An enhanced version 802.11e was released to introduce QoS support by differentiatingapplications based on their QoS requirements. This master thesis presents an overviewof 802.11 and its limitations in providing QoS support. It explains the enhanced version802.11e and how these limitations are overcome in it. This master thesis also discussesthe performance evaluation of 802.11 and 802.11e with the help of simulation scenariosin GloMoSim. The scenario results show the QoS support of 802.11e for different typesof data traffic due to its service differentiation technique.

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Acknowledgments

First of all, I am thankful to almighty Allah, who created me and gave me the courage tofulfill this task. I am also thankful to my loving parents and my family for their support,blessings and specially to my grandfather whose prayers and blessings are always withme.

I would like to express my heartiest gratitude to my study coordinator, Dr. PerLindstrom, for his guidance, help and cooperation.

Most importantly, I am also grateful to my supervisor, Dr. Thomas Nilsson, whoimpressed me a lot by his professional attitude, humorous nature and helping behavior.He has been always kind to me, guided me and encouraged me to understand theproblems while doing my thesis.

In the end, I am also grateful to my friends, Asrar, Imran and Waseem for their helpand unforgettable time during my studies in Sweden.

Muhammad Shahid Manzoor.

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Contents

1 Background of the Problem 1

1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Goal of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 An Overview of IEEE 802.11 3

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.1 Distributed Coordination Function (DCF) . . . . . . . . . . . . . 42.1.2 Point Coordination Function(PCF) . . . . . . . . . . . . . . . . . 52.1.3 How DCF Works: An Example . . . . . . . . . . . . . . . . . . . 5

3 Limitations of IEEE 802.11 regarding to Quality of Service 7

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4 An Overview of IEEE 802.11e 9

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.2 Hybrid Coordination Function (HCF) . . . . . . . . . . . . . . . . . . . 94.3 EDCA(Enhanced Distributed Channel Access) . . . . . . . . . . . . . . 10

4.3.1 ACs (Access Categories) . . . . . . . . . . . . . . . . . . . . . . . 104.3.2 Enhanced Distributed Channel Access Function (EDCAF) . . . . 104.3.3 Example of EDCA Access Mechanism . . . . . . . . . . . . . . . 11

5 Evaluation 13

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135.2 GloMoSim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135.3 DCF vs. EDCA Performance Comparison . . . . . . . . . . . . . . . . . 14

5.3.1 Scenario 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145.3.2 Scenario 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

6 Summary 19

6.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

v

vi CONTENTS

A List of Abbreviation 21

List of Figures

2.1 IEEE 802.11 System Architecture . . . . . . . . . . . . . . . . . . . . . . 32.2 DCF Access Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3 Example of DCF Mechanism . . . . . . . . . . . . . . . . . . . . . . . . 5

4.1 EDCA Access Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5.1 Throughput Comparison IEEE 802.11 and IEEE 802.11e, Scenario 1. . . 145.2 Aggregated Throughput Comparison IEEE 802.11 and IEEE 802.11e,

Scenario 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155.3 Delay Comparison of the IEEE 802.11e and IEEE 802.11, Scenario 1 . . 155.4 Throughput Comparison of the IEEE 802.11e and IEEE 802.11, Scenario 2. 165.5 Aggregated Throughput Comparison of the IEEE 802.11e and IEEE 802.11,

Scenario 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175.6 Delay Comparison of the IEEE 802.11e and IEEE 802.11, Scenario 2. . . 17

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viii LIST OF FIGURES

List of Tables

4.1 UP to AC Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.2 EDCA Parameter Values . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5.1 Data traffic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

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x LIST OF TABLES

Chapter 1

Background of the Problem

1.1 Background

The IEEE 802.11 wireless local area network (WLAN)nowadays is the most popularwireless technology of the world. It supports a data rate of 11 to 54 Mbps. Themedium access control (MAC) protocol of IEEE 802.11, called Distributed CoordinationFunction (DCF), is based on the carrier sense multiple access algorithm. The IEEE802.11 is becoming more and more popular due to its low cost and easy installation, butunfortunately it does not support Quality of Service (QoS). The Quality of Service refersto the capability of a network to provide data transfer with good quality. The QoS isoften measured in terms of the available bandwidth, delay in data transfer and data loss.Basically the MAC protocol of 802.11 treats all types of data traffic in the same way,on first come first served basis, regardless of the QoS requirements of the data traffic.The QoS requirements of the data traffic vary from application to application. Modernmultimedia applications are very sensitive to the available bandwidth, and delay in datatransfer. Some examples are, video and audio streaming, Internet telephony and on linenetwork games. This inability of 802.11 in providing QoS support is therefore a bighurdle in the success of multimedia applications over these networks. For this reason,a large amount of research work has been conducted in recent years to provide QoSsupport in IEEE 802.11 networks. An enhanced version of 802.11, called IEEE 802.11ehas been designed to provide QoS support.

1.2 Goal of Thesis

The master thesis presents a performance evaluation of the IEEE 802.11e and IEEE802.11 in order to support multimedia applications. The final draft of the IEEE 802.11ehas been released. The main task is to study what kind of enhancements made inIEEE 802.11e and understand the performance of IEEE 802.11e EDCA by comparing itwith the IEEE 802.11 DCF in order to support multimedia traffic. The IEEE 802.11eMAC utilizes a channel access function, called Hybrid Coordination Function, whichincludes both a contention-based channel access and a centrally-controlled channel accessmechanisms. The contention-based channel access is also called Enhanced DistributedCoordination Access (EDCA) and is a priority scheme. The goal of this thesis is toevaluate the performance of high priority traffic over these networks. Simulations will

2 Chapter 1. Background of the Problem

be used to compare EDCA and DCF mechanisms in GloMoSim [6].

1.3 Thesis Outline

Thesis report discusses the IEEE 802.11, its weaknesses in providing QoS support andhow QoS is introduced in the IEEE 802.11e. Thesis report is arranged as follows:Chapter 2 provides an overview of the IEEE 802.11 and its basic access mechanism.Chapter 3 discusses QoS limitations of IEEE 802.11. Section 4 presents an overview ofthe IEEE 802.11e, its access mechanism and how QoS is supported by introducing servicedifferentiation. Chapter 5 discusses about GloMoSim and the performance evaluation ofIEEE 802.11 and IEEE 802.11e, by comparing them with the help of real time simulationscenarios. Finally, the summary of the thesis report is discussed.

Chapter 2

An Overview of IEEE 802.11

2.1 Introduction

The IEEE 802.11 WLAN standard was released in 1997[1] by The IEEE (Institute ofElectrical and Electronics Engineers). It gained tremendous popularity after its release.It describes the specifications for the MAC and physical layer. It also describes differenttypes of physical layer specifications which are FHSS (Frequency Hopping Spread Spec-trum), DSSS (Direct sequence spread spectrum) and IR (Infrared). FHSS and DSSSphysical layers operate in the license free 2.4GHz ISM(Industrial, Scientific and Medi-cal) frequency band. These three layers support data transmission rates up to 2Mbps.In 1999, the IEEE introduced two new versions IEEE 802.11a[2] and IEEE 802.11b[3].The IEEE 802.11a operates on Orthogonal Frequency Division Multiplexing (OFDM)and provides a data rate from 11 to 54 Mbps in the 5GHz frequency band. The IEEE802.11b is based on DSSS. It operates in the 2.4 GHz frequency band and supports54 Mbps data transmission rate. The IEEE 802.11g[4] was launched by improving thephysical layer specifications of IEEE 802.11b in the 2.4 GHZ frequency band. The IEEE802.11g standard provides data rate of 54Mbps. Today, the IEEE 802.11 has becomean enormously popular wireless technology and has been deployed in offices, hotels andairports. It is incapable in providing QoS support to different application because ofno priority mechanism. The IEEE 802.11 standard comprises of two different basicstructures. Figure 2.1 illustrates the architecture of IEEE 802.11.

Figure 2.1: IEEE 802.11 System Architecture

4 Chapter 2. An Overview of IEEE 802.11

These structures are the Basic Service Set (BSS) and the Independent Basic ServiceSet (IBSS)[9]. A number of wireless stations are connected with an Access Point whichis responsible for communication between these stations. In an IBSS, wireless stationsare able to communicate with each other, within a provided transmission range. Thefundamental access methods are called the Distributed Coordination Function (DCF)and the Point Coordination Function (PCF) and are explained in the next section. Inthis section, a short overview of the DCF is described.

2.1.1 Distributed Coordination Function (DCF)

The DCF is the basic access mechanism of the IEEE 802.11 WLAN standard which isbased on the Carrier Sense Multiple Access (CSMA) algorithm and works as a listen-before-talk scheme. Listen before talk scheme means when a station senses the mediumbefore transmission. Figure 2.2 describes the DCF basic access mechanism. When astation senses and finds the medium idle for the DCF Inter frame Space (DIFS) timeperiod, the station will start the packet transmission. On the other hand, if the mediumis sensed busy during the DIFS time period by a station, it postpones its access to themedium and selects a random backoff value.

Figure 2.2: DCF Access Mechanism

The random backoff value is specified as the extra time, a station has to wait beforetrying to access the medium idle again after DIFS time period. If the station findsthe medium idle again, it will start decreasing the backoff time. During the backoffprocedure, if the medium becomes busy, the station will pause its backoff timer untilthe medium becomes idle again. When the backoff value becomes zero, the station cantransmit the frame. The random backoff time is used to prevent collisions. If two ormore stations find the medium idle and transmit their frames at the same time then acollision will occur. To prevent this situation, stations have to wait and choose a randombackoff time.

This technique is called Collision Avoidance (CA) and hence the whole mechanismis called CSMA/CA. When a receiver station receives a packet from the sender side, anACK frame is sent back to acknowledge the sender after the Short Inter Frame Space(SIFS) time period. The IEEE defined three IFS (Inter frame Space) time periodswhich are SIFS, DIFS in DCF and Point Inter Frame Space (PIFS) in PCF to controlthe medium access. The DIFS is the largest IFS and SIFS is the shortest IFS. Relyingon the priority of the frame exchange sequence, continuous frame transmissions can bedivided by these inter frame spaces. Greater the priority of a frame exchange sequencethe shorter the inter frame space used between frames. The random backoff value is

2.1. Introduction 5

selected uniformly from the range (0, CW) where CW is called Contention Window.The contention Window (CW) is set to minimum CWmin and becomes doubled everytime a transmission fails, until it reaches its maximum size CWmax.

It will reset again after every successful transmission. Physical Carrier Sensing andVirtual Carrier Sensing are used to investigate the access to the medium. Virtual CarrierSensing is used at the MAC layer. When a station receives a frame at MAC layer whichis not addressed to itself, it will observe the time from the frame header where the frameheader explains the time require for the transmission of the frame. Then it defers themedium access for that particular time period. On the other hand, with Physical CarrierSensing, the station senses itself at physical layer[10, 8].

2.1.2 Point Coordination Function(PCF)

Point Coordination Function (PCF) is also an access mechanism of the IEEE 802.11which is based on centrally controlled medium access. The AP works as a coordinatorcalled the PC (Point Coordinator) and provides contention free channel access to themedium for individual stations by polling them for transmissions.

2.1.3 How DCF Works: An Example

In this Figure 2.3, the DCF mechanism is illustrated with the backoff procedure. STA1,STA2 and STA3 stations are contending for the medium. As shown in the figure, STA1senses the channel and the medium is found idle. STA1 starts the packet transmission,in the meantime STA2 and STA3 try to sense the medium for transmission but find themedium busy. Both stations defer their access and waits for a complete exchange oftransmission (DATA +SIFS +ACK).

Figure 2.3: Example of DCF Mechanism

After the transmission is finished, the medium becomes idle. All stations wait fora DIFS time period and then they select the random backoff value. The figure depictsthat the values selected by STA1, STA2 and STA3 are 13, 2, and 8 respectively. Now,the backoff procedure starts to decrementing the backoff value. The STA2 reaches zeroand wins the channel access for transmission.

STA1 and STA3 pause their access and waits for an idle medium. When mediumbecomes idle again, the stations choose the backoff values after a DIFS time period. Theselected backoff values of STA1, STA2 and STA3 are 11, 6, and 6 for this time. Here,STA2 and STA3 try to transmit the packet at the same time after the backoff valuesreache to zero, which leads to a collision as shown in the figure 2.3.

6 Chapter 2. An Overview of IEEE 802.11

STA2 and STA3 do not know about the collision, and wait for an ACK. Since noACK is received before the ACK time out, both stations assume that a collision hashappened and double their CW values. The chances of STA1 of winning the mediumaccess increases because of not doubling its CW value. It proceeds counting from itspaused state and so on.

Chapter 3

Limitations of IEEE 802.11regarding to Quality of Service

3.1 Introduction

The QoS is a networking term which describes a set of properties like bandwidth use,jitter, delay, packet loss, and throughput. The QoS is stated as quality of the datatraffic over a network. Since, QoS requirements differ from application to applicationand all applications have particular QoS requirements. A network which is capable offulfilling each application’s QoS requirements is called QoS supported network. TheQoS requirements are organized in three types such as bandwidth, delay and data loss[7, 11, 10].

1. Bandwidth: Bandwidth is an important parameter. The amount of data that canbe transfered during a given time period is referred to the Bandwidth. On receivinghigh bandwidth, applications are able to deliver data in large amount. Bandwidthsensitive applications are those application which require consistent data rate andany change in bandwidth may cause in the form of data loss and unnecessarydelays. Some examples of Bandwidth Sensitive applications are Internet telephonyand video conferencing.

2. Data Loss: Elastic applications such as email, web pages and file transfer areusually classified as loss tolerant applications. These applications are capable totolerate low bandwidth and undesired delays but demand a constant transfer ofdata. Multimedia applications are organized as bandwidth and delay sensitive butusually are loss-tolerant. These applications require constant bandwidth and delayassurance. These application can severely be affected due to data or packet losses.

3. Delay: Multimedia applications are considered as delay sensitive applications.Some examples are video conferencing, Internet telephony and VOIP (Voice overIP). These application can be suffer due to an increase in delay. Therefore, strictconstraints are applied on these applications in terms of delay and bandwidth.

The IEEE 802.11 standard serves as best effort service model. All kinds of data trafficis treated on first come and first served basis regardless of their QoS requirements. It cannot classify applications on the basis of their QoS requirements. The bandwidth sensitive

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8 Chapter 3. Limitations of IEEE 802.11 regarding to Quality of Service

applications have priority over delay sensitive applications regarding to bandwidth. Theinability in providing QoS is a big barrier in the success of 802.11. Currently, the IEEE802.11e standard has been launched which is called QoS supported network.

Chapter 4

An Overview of IEEE 802.11e

4.1 Introduction

To provide quality of service (QoS) support, the IEEE carried out a large amount ofresearch work on the IEEE 802.11 MAC. Then a newly improved version IEEE 802.11ewas released [5]. A priority mechanism has been introduced in the enhanced version tofacilitate the QoS support. It deals with each type of data traffic according to their QoSrequirements. Applications can be categorized in four Access Categories (AC) on thebasis of their QoS requirements. Every frame with a specific priority of data traffic isthen assigned to one of these access categories. For each AC, service differentiation isdefined by utilizing a different set of contention parameters to get the medium access.In IEEE 802.11, QoS Access Point (QAP) and QoS station (QSTA) provide QoS andalso the Basic Service Set is called QoS Basic Service Set (QBSS).

4.2 Hybrid Coordination Function (HCF)

HCF is the latest centralized coordination function introduced by the IEEE 802.11e. Itcombines the features of Distributed medium access like DCF and centrally controlledmedium access like PCF with improved QoS techniques. HCF defines two types of ac-cess mechanisms, The distributed contention-based channel access mechanism is calledEDCA (Enhanced Distributed Channel Access) and the centrally controlled contention-free access mechanism is called HCCA (HCF Controlled Channel Access). In this chap-ter, the focus is on the EDCA.

10 Chapter 4. An Overview of IEEE 802.11e

4.3 EDCA(Enhanced Distributed Channel Access)

The EDCA mechanism is an enhanced version of the DCF mechanism, which providesdistributed medium access with the help of access categories (ACs).

4.3.1 ACs (Access Categories)

The EDCA describes four ACs to handle with several types of data traffic. The fouraccess categories AC BK, AC BE, AC VI, and AC VO are introduced for Voice, Video,Best Effort and Background. Frames are then mapped according to their QoS require-ments on their particular AC’s where AC Vo and AC BK have the highest and thelowest priority respectively.

When a frame reaches the MAC layer, it has a certain priority value which is calledUser Priority (UP). User Priority of the frame is mapped to its related AC. There areeight different priorities assigned to ACs, which are listed in Table 4.1.

AC UP Priority

AC BK 1 Lowest

AC BK 2 .

AC BE 0 .

AC BE 3 .

AC VI 4 .

AC VI 5 .

AC VO 6 .

AC VO 7 Highest

Table 4.1: UP to AC Mapping

4.3.2 Enhanced Distributed Channel Access Function (EDCAF)

EDCAF is an improved version of DCF which contends for the medium access on thebasis of the specified parameters of an AC. EDCAF parameters which are related to anAC for the medium access are as follows[10]:

1. Arbitration Inter Frame Space (AIFS) AIFS is the minimum time period duringwhich the medium is found idle before transmission of frames. The followingequation is used to derive the AIFS.

AIFS=AIFSN x aSlotTime + aSIFSTime

Where AIFSN(Arbitration Inter Frame Space Number) refers to length of theAIFS, SlotTime refers to the slot time and SIFSTime is the SIFS time period.Higher priority ACs have to wait for less time before starting transmissions becauseof their smaller AIFS values. In the case of low priority ACs, they have to wait forlonger time and may suffer from long delays because of their higher AIFS values.

2. CWmin and CWmax: The maximum and minimum contention window size variesbetween ACs. Lower priority ACs have larger CWmin and CWmax values while

4.3. EDCA(Enhanced Distributed Channel Access) 11

higher ACs have smaller CWmin and CWmax values. Therefore, for an AC witha small contention window, the EDCAF related to that particular AC will picka small random backoff value. In this way, the EDCAF has to wait for a veryshort AIFS time period when medium becomes idle. The CWmin values for thehigh priority ACs are half or quarter of the low priority ACs. As high priorityACs select small backoff values, there are very short delays to access the medium.Due to the small CWmin sizes, high priority ACs experience a large number ofcollisions. Table 4.2 expresses the EDCA parameter values.

ACs CWmin CWmax AIFSN

AC BK 31 1023 7

AC BE 31 1023 3

AC VI 15 31 2

AC VO 7 15 2

Table 4.2: EDCA Parameter Values

4.3.3 Example of EDCA Access Mechanism

Figure 4.1 illustrates the EDCA access mechanism. The EDCA access mechanism hasdifferent sets of parameters for different ACs. When the medium becomes idle for anAIFS time period, the EDCAF selects a random number called backoff (BO) valueand begins decreasing the backoff timer. When the backoff timer reaches zero, thetransmission is started.

Figure 4.1: EDCA Access Mechanism

Considering figure 4.1, we can see that the high priority ACs, AC VO and AC VIhave smaller values of AIFS and contend for the medium more quickly compared to thelow priority ACs, AC BE and AC BK. This is the first benefit the high priority ACshave. Secondly high priority ACs also have smaller CW sizes as compare to low priorityACs. According to the sizes of CWs, a random number is selected (BO). In the contextof small values of AIFS and BO, the high priority ACs have more access to the mediumas compared to the low priority ACs. This means, that high priority ACs achieve a largebandwidth share.

12 Chapter 4. An Overview of IEEE 802.11e

In figure 4.1, the high priority ACs choose small BO values and reach zero first andstart the transmission where low priority ACs pause their backoff timers. Every timeEDCAF of AC VO and EDCAF of AC VI choose small BO from the fixed CW sizes,’7’ and ’15’, respectively. High priority ACs EDCAF win the medium access and lowpriority ACs suffer from their large AIFS time periods.

The selection of different BOs from different CW sizes reduce the probability ofcollisions among stations. But still there is a possibility that the EDCAF of low priorityAC and EDCAF of high priority AC choose the same BO values, which causes a collision.

Chapter 5

Evaluation

5.1 Introduction

This chapter presents a short overview of the GloMoSim [6]. It also describes theperformance comparison of IEEE 802.11 and IEEE 802.11e in order to support QoSrequirements of different applications. The data traffic types and their properties areshown in the Table 5.1. The evaluation of the EDCA and DCF performance is observedby considering some real time scenarios. The performance metrics used are stated below.

– Throughput: The amount of data delivered over a network in a specific time. Itis directly proportional to the available bandwidth/capacity and is shown in bitsper second.

– Delay: It is measured from the time at which a sender sends a packet to the timeat which a receiver receives the packet.

– Aggregated Throughput: It is the throughput of the whole network.It is measuredby addition of throughputs of different traffic streams.

ACs pkt Size (Bytes) Rate (Kbps) Interval (ms)

AC BK 1024 76 107.8

AC BE 1024 112 73.1

AC VI 1460 96 122

AC VO 80 28 22

Table 5.1: Data traffic properties

5.2 GloMoSim

Global Mobile Information System Simulator (GloMoSim) is a scalable network simu-lation environment developed at the UCLA Parallel Computing laboratory. By usingparallel execution, GloMoSim achieves scalability to reduce simulation time. The Glo-MoSim is written in PARSEC, a C based language and is an event discrete simulator.

14 Chapter 5. Evaluation

It is built by using a layered architecture approach similar to the OSI (Open SystemsInterconnection) layered architecture. It works as a powerful simulator, that can sim-ulate a wide range of protocols and models at different layers. Parallel and sequentialexecutions of event discrete simulations are supported as well.

The simulation depends on handling discrete events. Execution contains a set ofevents and an event causes a change in the state of the system. A particular event orcombination of events may activate other events and so on, this is how the simulationproceeds. A node movement, a packet reception can be an event. It is free for educationalpurposes but supports only the sequential simulations.

5.3 DCF vs. EDCA Performance Comparison

5.3.1 Scenario 1

A simple scenario is considered in order to provide a comparison of the IEEE 802.11DCF and the IEEE 802.11e EDCA. The aim is to observe the performance of individualACs and how data traffic related to a specific AC is treated and also to evaluate theperformance of the IEEE 802.11e EDCA and 802.11 DCF. Just for revision, in the IEEE802.11e EDCA, all traffic streams are served related to their ACs. There is no prioritymechanism in the IEEE 802.11 DCF, all traffic streams are treated with the same way.The number of stations are used from 1 to 30 which are transmitting all four type ofdata traffic. In this scenario, there are three station for the high priority traffic streams(Voice and Video). The rest of the stations are for the Low priority traffic (Best Effortand Background), and are increased gradually from 4 to 30. The performance metricesare considered aggregated Throughput, throughput and delay.

Results

It is obvious from the results in figure 5.1 that the 802.11e EDCA defines service differ-entiation successfully through different ACs while the IEEE 802.11 DCF serves all kindsof traffic streams in the same way.

Figure 5.1: Throughput Comparison IEEE 802.11 and IEEE 802.11e, Scenario 1.

Figure 5.1 depicts that, in the IEEE 802.11, throughput of high priority trafficstreams are consistently decreased and the throughput of low priority traffic streams

5.3. DCF vs. EDCA Performance Comparison 15

are increased continuously with the addition of low priority stations. In IEEE 802.11e,due to its service differentiation mechanism, throughput of high priority traffic streamsremains constant, as there is no effect of change throughput with the addition of lowpriority stations. Concerning to low priority traffic streams, the throughput of AC BE issharply increased up to station 12 and after that it is decreased slowly. The throughputof AC BK is also increased to the 10th station but severely dropped at the 12th station.After the 12th station, the throughput has started decreasing.

Figure 5.2: Aggregated Throughput Comparison IEEE 802.11 and IEEE 802.11e, Sce-nario 1.

The aggregated throughput results in figure 5.2 shows that 802.11e provides improvedthroughput as compared to IEEE 802.11. Considering the scenario, the IEEE 802.11shows stable aggregated throughput but IEEE 802.11e throughput starts decreasing.

Figure 5.3: Delay Comparison of the IEEE 802.11e and IEEE 802.11, Scenario 1

Delay results are presented in figure 5.3, which describe that the high priority trafficsuffer more delays compared to low priority traffic in the 802.11 due to the lackness of theQoS support mechanism. AC VI experiences delay which starts increasing subsequentlyand AC VO also suffers delay which increases afterward, but low priority traffic donot suffer as high priority traffic do. In the IEEE 802.11e, AC VO and AC VI (HighPriority Traffic) do not suffer longer delays because of their CWmin and CWmax values

16 Chapter 5. Evaluation

as compared to AC BE and AC BK (Low Priority Traffic). The delay for AC BK beginsincreasing, but at the same time, AC BE experiences a very mall delay.

5.3.2 Scenario 2

Scenario 2 is similar to Scenario 1, except that initially there are first 6 station for Voiceand Video traffic. The number of stations in this scenario are increased from 7 to 60 andreserved for best effort and background traffic. First six stations only transmit Video andvoice traffic, but rest of the stations transmit best effort and background streams. themain aim is to observe the performance of high priority traffic with increasing numberof stations of low priority traffic. The performance metrices studied are throughput,aggregated throughput and delay.

Results

Figure 5.4: Throughput Comparison of the IEEE 802.11e and IEEE 802.11, Scenario 2.

Figure 5.4 shows the throughput results for IEEE 802.11 and IEEE 802.11e to com-pare the results of traffic streams with different bit rates. The figures 5.4 show theefficiency of the IEEE 802.11e QoS mechanism clearly that Voice and Video trafficstreams achieves consistent throughput and there in no effect of addition in best effortand background ACs. While in IEEE 802.11, best effort and background traffic startsto increase and voice and video traffic starts to drop.

Figure 5.5 shows aggregated throughput results of IEEE 802.11 and IEEE 802.11e.In the IEEE 802.11e, aggregated throughput is less than 1 Mbps, but in the IEEE802.11, it is more than 1 Mbps.

Here we can observe from the Figure 5.6 that in IEEE 802.11, due to increase in lowpriority stations, voice and video traffic suffer from longer delays but low priority trafficsuffer from very small delays. From the results, we can examine that in 802.11e, delaysfor voice and video traffic stay under satisfactory limits while best effort and backgroundstreams experience longer delays.

5.3. DCF vs. EDCA Performance Comparison 17

Figure 5.5: Aggregated Throughput Comparison of the IEEE 802.11e and IEEE 802.11,Scenario 2.

Figure 5.6: Delay Comparison of the IEEE 802.11e and IEEE 802.11, Scenario 2.

18 Chapter 5. Evaluation

Chapter 6

Summary

6.1 Summary

The IEEE 802.11 standard was released in 1997. It has become most well known WLANtechnology due to its simplicity, ease of deployment and low cost. Three versions of802.11 were also introduced to support maximum data rates of 11 to 54 Mbps. Thetwo fundamental access mechanisms of the IEEE 802.11 are called DCF and PCF. TheDCF is a distributed medium access mechanism based on CSMA while in PCF, accessto medium is centrally controlled by the AP. In DCF, a station senses the medium andif it is found idles for DIFS time period, it starts the transmission.

When the medium becomes busy, it chooses a random BO value and waits for themedium until it becomes idle again. The random BO value is selected in the range of(0, CW), where CW is called the contention window. The limitation in the IEEE 802.11DCF is that, it does not provide QoS support. The QoS is the capability of a networkto provide data traffic with good quality. The term QoS defines a set of quantitativeand qualitative characteristics like bandwidth, delay and data loss.

The QoS requirements vary from application to application. Unfortunately, In DCF,all type of applications are served on a first come and first serve basis. The IEEE802.11 is unable to differentiate between the applications on the basis of their QoSrequirements. A recently released version of the IEEE 802.11 is called the 802.11e,which provides support for QoS by introducing a service differentiation mechanism.Four ACs are defined to serve applications. Data traffic from different applications aremapped to these access categories on the basis of their QoS requirements. The EDCAuses different medium access parameters for different access categories to support theQoS.

This thesis also concludes a performance comparison between the EDCA and theDCF mechanisms in order to support QoS requirements for different type of data traffic.The results show that all types of data traffic are treated equally in the IEEE 802.11DCF which causes lackness in QoS support. Conversely, the IEEE 802.11e treats alldata traffics on the basis of their QoS requirements and priority. Two scenarios aresimulated in order to compare EDCA and DCF performance concerning to support theQoS. There are 30 stations and then 60 stations have been taken in scenario 1 andscenario 2 respectively. In the 1st scenario, initially 3 stations are transmitting voiceand video traffic and the remaining stations are transmitting low priority traffic of besteffort and background traffic.

20 Chapter 6. Summary

Similarly, in second scenario, first 6 stations are transmitting high priority trafficand rest of them are transmitting low priority traffic. It is very clear from the resultsthat EDCA performs better and provides service differentiation mechanism for differenttypes of data traffic, while lacking of priority mechanism, DCF serves each application inthe same way apart from their QoS requirements. There are three performance metricsare used to evaluate the performance of EDCA and DCF which are throughput, delayand aggregated throughput.

Appendix A

List of Abbreviation

Abbreviation Designation

AC Access Category

AC VO Access Category Voice

AC VI Access Category Video

AC BE Access Category BestEffort

AC BK Access Category BackGround

AIFS Arbitration Inter Frame Space

AP Access Point

BO Back off

CSMA/CA Carrier Sense Multiple Access / Collision Avoidance

CW Contention Window

CWmax Contention Window Maximum

CWmin Contention Window Minimum

DCF Distributed Coordination Function

DIFS DCF Inter Frame Space

DSSS Direct Sequence Spread Spectrum

EDCA Enhanced Distributed Channel Access

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22 Chapter A. List of Abbreviation

EDCAF Enhanced Distributed Channel Access Function

GloMoSim Global Mobile Information System Simulator

HCCA HCF Controlled Channel Access

HCF Hybrid Coordination Function

IBSS Independent Basic Service Set

IEEE Institute of Electrical and Electronics Engineers

IP Internet Protocol

MAC Medium Access Control

Mbps Mega bit per second

OFDM Orthogonal frequency-division multiplexing

PCF Point Coordination Function

QoS Quality of Service

SIFS Short Inter Frame Space

STA Station

UCLA University of California, Los Angeles

WLAN Wireless Local Area Network

References

[1] IEEE Std. 802.11, Part 11: Wireless LAN Medium Access Control (MAC) andPhysical Layer (PHY) Specifications. 1997.

[2] IEEE Std. 802.11a, Supplement to Part 11: Wireless LAN Medium Access Con-trol (MAC) and Physical Layer (PHY) Specifications: Higher-Speed Physical LayerExtension in the 5 GHz Band. 1999.

[3] IEEE Std. 802.11b, Supplement to Part 11: Wireless LAN Medium Access Con-trol (MAC) and Physical Layer (PHY) Specifications: Higher-Speed Physical LayerExtension in the 2.4 GHz Band. 1999.

[4] IEEE Std. 802.11g, Supplement to Part 11: Wireless LAN Medium Access Control(MAC) and Physical Layer (PHY) Specifications: Further Higher-Speed PhysicalLayer Extension in the 2.4 GHz Band. 2003.

[5] IEEE 802.11e/D13.0, Draft Supplement to Part 11: Wireless LAN Medium AccessControl (MAC) and Physical Layer (PHY) Specifications: Medium Access Control(MAC) Quality of Service (QoS) Enhancements. January 2005.

[6] R. Bagrodia and X. Zeng. Glomosim, A Library for the Parallel Simulation of LargeWireless Networks. Proceedings of the 12th Workshop on Parallel and DistributedSimulation (PADS 98), pages 154–161, 1998.

[7] Albert Banchs, Arturo Azcorra, Carlos Garcia, and Ruben Cuevas. Applicationsand Challenges of the 802.11e EDCA Mechanism: An Experimental Study . In IEEENetwork, volume 19, July 2005.

[8] Giuseppe Bianchi. Performance Analysis of the IEEE 802.11 Distributed Coordi-nation Function. In IEEE JSAC, volume 18, pages 535–547, March 2000.

[9] D.J. Deng and R.S. Chang. A priority scheme for IEEE 802.11 DCF access method.IEICE Trans. Commun, 82:96–102, 1999.

[10] Jahanzeb Farooq and Bilal Rauf. Implementation and Evaluation of IEEE 802.11eWireless LAN in GloMoSim. Department of Computing Science, Umea University,Umea, Sweden, 2006.

[11] Thomas Nilsson. Resource Allocation and Service Differentation in Wireless LocalArea Networks. Licentiate Thesis, Dept. of Computing Science, Umea University,June 2005.

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